The present invention and its underlying problem will hereinafter be described on the basis of recent research activities of multiple authors, however, without restricting the invention to this sort of application.
M. Melon, et. al. investigated alternative methods in the paper “Comparison of four subwoofer measurement techniques,” J. Audio Eng. Soc. 55 (12), 1077-1091 (2007). At high frequencies the direct sound wrad generated by a source under test Q0 can be separated by windowing the impulse response from a later arriving room reflections wref. This technique is not applicable at low frequencies where the corresponding wave length is not small compared with the minimum distance between source Q0 and surrounding boundary surfaces.
G. Weinreich and E. Arnold suggested in the paper “Method for measuring acoustic radiation fields,” published in the J. Acoustical Soc. Am. 68(2), 404-411 (1980), a holographic Input Output Field Separation Method (IOFS) for measuring the direct sound of the source under test Q0 based on an expansion of the sound pressure measured at two concentric spheres.
Melon applied this IOFS-method to loudspeakers in the paper “Measurement of subwoofers with the field separation method: comparison of p-p and p-v formulations,” published in proceedings of the Acoustics 2012 Conference, 23-27 Apr. 2012, Nantes, France. The sound pS,scatQ0+Q1 reflected and scattered at the surface S0 of the source under test Q0 causes an error in the measurement if the surface S0 is not small compared to size of the scanning surface S1.
To overcome this problem C.-X. Bi suggested in the paper “Recovery of the free field using the spherical wave superposition method”, published in Proceedings of the Acoustics 2012 Conference, 23-27 Apr. 2012, Nantes, France, a holographic Primary Source Field Separation Method (PSFS) using the acoustical impedance YS0 and the shape of the surface S0 as additional input information. Due to the complexity of the shape and variety of materials used in loudspeaker design those information are not easily available and the method is usually not applicable in practice.
C. Langrenne, suggested in his thesis “Methodes de regularisation du probleme inverse acoustique pour l'indentification de sources en milieu confine et pertube, Universite du Maine, 1997, a perturbation method for separating direct sound from the wave reflected and scattered at surface S0. This method requires additional measurements of the sound pressure pt(rm) at multiple measurement points rm with m=1, . . . M while deactivating the source under test Q0 and activating a perturbation source Q2(re) at multiple positions re with e=1, . . . , E in the space between outer scanning surface S2 and the room boundary SB. The perturbation technique is very time consuming because the determination of the transfer matrix H requires a large number of measurements (product of E and M).
The known field separation methods using spherical harmonics are limited to frequencies below a cut-off frequency fG≈55 N/r1, where the maximal order N of the expansion is high enough to approximate the field enclosed by a spherical scanning surface S1 with the radius r1.
Melon scans the sound pressure on a spherical surface in the half space by placing the source under test on an acoustically hard floor. The center of the scanning surface corresponds with the origin of the spherical coordinate system and is used as the expansion point r0 in the wave expansion based on spherical harmonics. Thus the expansion point r0 is not identical with the acoustical center of the source under test. The holographic measurements of loudspeaker systems using multiple transducers mounted in a large enclosure require a high order N of the wave expansion associated with a large number of measurement points to describe the sound pressure in the near field of the source at sufficient accuracy.
There is a need to find a simpler/more reliable possibility to measure the direct sound of a source under test Q0 under the influence of room reflection and ambient noise which overcomes the disadvantages of the known methods.